Physics: Elastic Powered Car

[kingwinner]

Hi, I am in intro physics and have to build an elastic powered car that will be as small as possible, can build quickly and will travel as far as possible. I need a box with all the parts in it. I can use the wheels and axis that are pre-made. After the timer starts timing, I have to re-build my car as fast as possible, setting up the elastic system, and then release the car. Once the car STOPS, the timer will stop timing. The score is calculated using the formula Score = [(volume of box) * (time taken)] / (distance travelled) with the lowest score being the goal. So I think I need a car that can move quickly and stop quickly (since the teacher will continue timing even after I released the car) while travelling a very long distance.

 

I have already spent a day to make a rubber band car, this is how it looks like:

 

Top view:

http://www.geocities.com/asdfasdf23135/DSC00707.JPG

 

Bottom view and the wheels:

http://www.geocities.com/asdfasdf23135/DSC00708.JPG

 

I used 4 CD wheels, 2 thin axes and a thick rubber band. (I cut the rubber band so it's like a single stretch.) I used tape to stick one end of my thick rubber band on the rear axis and the other end in the front part of my car. It runs quite well on certain surfaces but when I run it on carpet and some smooth surfaces, it is NOT moving, the back wheels are rolling but on the same spot (i.e. not moving). I tried to adjust many different things but I still can't figure out why...and also, I have to roll my car backward for a very long time for it to travel only a short distance. Why? Would a thicker axis improve this? Furthermore, my car is not travelling in a straight line, how can I improve this?

 

Will it get a better performance overall based on the “score formula” if I use:

-a thick rubber band?

-a long rubber band?

-smaller wheels?

-larger axis(in diameter)?

-2 or more rubber bands?

 

I really appreciate for your help! Thanks a million!

[kingwinner]

There is one very important question that I would like to ask to continue working on my car:

 

Provided there are 2 identical cars, but the first car has CD wheels WITHOUT rubber bands warpping around it and the second car has CD wheels WITH rubber bands warpping around all wheels.

 

The second car has better traction. My question is, would higher traction (frction of wheels with the ground) increase or decrease the speed and travel distance of my car? Higher traction means higher friction, right? (and friction tends to stop things) What are the advantages and disvantages of having high traction wheels in such a competition like mine.

 

Thank you!

[TheBigDog]

There is one very important question that I would like to ask to continue working on my car:

 

Provided there are 2 identical cars, but the first car has CD wheels WITHOUT rubber bands warpping around it and the second car has CD wheels WITH rubber bands warpping around all wheels.

 

The second car has better traction. My question is, would higher traction (frction of wheels with the ground) increase or decrease the speed and travel distance of my car? Higher traction means higher friction, right? (and friction tends to stop things) What are the advantages and disvantages of having high traction wheels in such a competition like mine.

 

Thank you!

King, I am not going to give you answers, but I will recommend that you perform a DOE (Design of Experiment). This is a method of determining the effects of multiple variables on a peocess. You have described the following variables.

 

-a thick rubber band?

-a long rubber band?

-smaller wheels?

-larger axis(in diameter)?

-2 or more rubber bands?

 

Break them down into pairs if you can.

 

Rubber bands = 1 or 2

Rubber bands = thick or thin

Wheel diameter = big or small

Axis (axel?) diameter = big or small

 

For this experiment you would need to run 2^4 tests(4 variables, two conditions each). They would include every combination possible. The outcome of the tests will provide the data to answer all of your questions. Keep close track and use keen observation.

 

Test 1 = 1 band, thick, big wheels, big axel

Test 2 = 1 band, thick, big wheels, small axel

Test 3 = 1 band, thick, small wheels, big axel

Test 4 = 1 band, thick, small wheels, small axel

Test 5 = 1 band, thin, big wheels, big axel

Test 6 = 1 band, thin, big wheels, small axel

Test 7 = 1 band, thin, small wheels, big axel

Test 8 = 1 band, thin, small wheels, small axel

Test 9 = 2 bands, thick, big wheels, big axel

Test 10 = 2 bands, thick, big wheels, small axel

Test 11 = 2 bands, thick, small wheels, big axel

Test 12 = 2 bands, thick, small wheels, small axel

Test 13 = 2 bands, thin, big wheels, big axel

Test 14 = 2 bands, thin, big wheels, small axel

Test 15 = 2 bands, thin, small wheels, big axel

Test 16 = 2 bands, thin, small wheels, small axel

 

You are never to your to use Six Sigma methods!

 

Bill (Black Belt)

[kingwinner]

The problem is I don't have that many materials, for instance, I don't have a large axis and another frame. And I don't have small wheels that can be attached to an axle. The only thing that I can try is using two rubber bands streatching at the same time (which I tried), but it doesn't work well because sometimes when one rubber band unwinded completely, the other is not, or one is stuck...

 

I will have to buy some materials with a limited budget, so it would be nice if someone can give me some hints!:ebaskbal: Currently, I am having a problem on traction. Would higher traction (i.e. higher friction?) wheels always be the better choice against a set of lower traction wheels? If there is lower traction (and thus lower friction), wouldn't the car travel faster and further?

[TheBigDog]

The problem is I don't have that many materials, for instance, I don't have a large axis and another frame. And I don't have small wheels that can be attached to an axle. The only thing that I can try is using two rubber bands streatching at the same time (which I tried), but it doesn't work well because sometimes when one rubber band unwinded completely, the other is not, or one is stuck...

 

I will have to buy some materials with a limited budget, so it would be nice if someone can give me some hints!:ebaskbal: Currently, I am having a problem on traction. Would higher traction (i.e. higher friction?) wheels always be the better choice against a set of lower traction wheels? If there is lower traction (and thus lower friction), wouldn't the car travel faster and further?

The answer lies in your drive train. Think about how slippage effects the distance your car travels.

[kingwinner]

Since I have to re-build everything on the day of the competition, so I can't really use glue? I need something that can be detached and assambled quickly......How can I make the rubber band and the axle to stick together and roll together? A hook may do this, but there is no way I can pierce the metal axle with a hook and if there is a hook on the axle, I can't take the axle out of my car frame quickly...

 

And for a rubber band car, would a heavier or lighter car be better to increase the speed and travel distance?

[kingwinner]

I have tried to glue a hook to the rear axle to catch and release the rubber band to maximize the travel distance, it works really well, the rubber band is released after unwinding...

 

But I have to re-assemble the axle, wheels, and the elastic system on the competition day and will be evaluated on (the less assemble time, the better the score), so if I glue the hook to the axle, I wouldn't be able to slide the axle into the frame becuase the hook prevents to do so. How can I deal with this problem?

 

Besides, would it be better to make the rubber band car by stretching the rubber band to store energy instead of winding the rubber band around the axle? Would it work (stores energy by stretching) if I tie one end of a string to the rubber band and the other end to the rear axle?

 

Thank you for your reply! :naughty:

[kingwinner]

I have developed another simpler way of releasing the rubber band after unwinding, I can simply hold the rubber band and wind the first few wraps around the axle using my hand, and then keep winding the rear wheels, after that, the car goes and the rubber band will fly out (released from the rear axle) and I can see a great increase in travel distance :) My car can travel around 8m for now! I have tried my very best already, but I want it to go further

 

I have made a car with the frame size around 10cm x 8cm x 4cm, this is a short car (not long), and when I stretch the rubber band, the wheels are spinning off sometimes and goes upsidedown, should I add some heavy weight to prevent this from happening? Would this heavy weight affect the car's performance in speed and distance?

 

I feel that small volume car can't go as far and as fast as a larger volume car! Is there a reason for this? How can I make a short, small volume rubber band car that can travel more than 15 metres and moves fast?